1. Field of the Invention
The present invention relates to a sheet feeding apparatus for feeding a sheet by driving a sheet feeding mechanism by means of a stepping motor, and a facsimile system having such sheet feeding apparatus.
2. Related Background Art
FIG. 1 shows a schematic construction of a facsimile system wherein a recording sheet 60 comprised of a heat-sensitive paper which is colored when heat is applied thereto is fed from a sheet roll 61 of a continuous long sheet material housed in a roll holder 62. An image is recorded on the recording sheet 60 in a recording portion 63, and the recorded sheet is cut from the remaining sheet 60 by means of a cutter 64 and then is ejected or discharged by ejecting rollers 65 out of the system. Further, in an original reading portion 66, original sheets stacked on an upper cover are separated one by one by means of separating rollers 6a, and the separated original is moved at a constant speed by means of a feed roller 6b. Meanwhile, a light source 6c illuminates the original, and the reflected light from the original is sent to a photoelectric converter element 6f through a mirror 6d and a lens 6e, so that the original information is converted to an electric signal which is to be sent to the predetermined recording portion 63 or a memory (not shown).
Now, a recording operation performed in the recording portion 63 will be explained. The recording portion 63 includes a line-type recording head which comprises a plurality of heating elements selectively heated in response to an image signal and which is urged, by means of a bias spring 69, against a platen roller 67 acting as feeding means for the recording sheet 60. By rotating the platen 67 in a direction shown by an arrow a through a movement transmission system (not shown) by means of a motor 69, the recording sheet 60 is fed, and the image is thermally recorded on the recording sheet 60 by heating the recording head 68 in response to the image signal.
On the other hand, a side wall 70 which separates or partitions the roll holder 62 from the recording portion 63 is designed to have a height of about 1/2 of a maximum permissible diameter of the sheet roll, and a space 71 is provided between the side wall 70 and the platen roller 67. The space 71 serves to create a gentle slack in the leading portion of the recording sheet 60 when the leading edge of the recording sheet 60 is returned from a cutting position of the cutter 64 to a position where the leading edge of the sheet is pinched or nipped between the platen roller 67 and the recording head 68. To this end, the space 71 has a width (between the sheet roll and the platen roller) to an extent that the recording sheet 60 is not bent at an acute angle when the sheet is slackened. Incidentally, the recording sheet 60 is guided toward the recording portion 63 by means of a guide member 72.
A recent facsimile system has transmission-reception function through memory means of large capacity, and includes multiple address function, repeater function, confidential function or the like to improve the operability and efficiency thereof. To achieve this, techniques regarding the high speed reading of the original and/or the high speed recording have been required, and, particularly, a method for driving motors such as a motor for feeding the original and a motor for feeding the recording sheet have been called to account.
Now, FIG. 2 shows, in a conventional facsimile system, a damping amount of a four phase unipolar stepping motor for rotatingly driving the platen roller 67 in the recording portion 63 or the feed roller 6b in the original reading portion 66, during first phase excitation and second phase excitation thereof, when such stepping motor is driven at a low speed with 1-2 phase excitation. FIG. 3 shows a displacement amount of the feed roller 6b, when the stepping motor of FIG. 2 is driven at a low speed with 1-2 phase excitation.
As seen in FIG. 2, in this stepping motor, the damping occurs when the first excitation is changed to the second excitation by trigger. Consequently, the movement of the feed roller 6b becomes non-uniform or uneven, as shown in FIG. 3, and, thus the original will be fed unevenly due to the trigger. Further, since the feed roller 6b rotates only in one direction, when a considerable backlash occurs in the motor as shown in FIG. 2, a maximum displacement ("P" in FIGS. 2 and 3) of the motor in a positive direction (upward direction in FIGS. 2 and 3) leads to the rotational amount of the feed roller 6b in FIG. 3 just as it is, thus resulting in the error of reading in the positive direction.
FIG. 4 shows a displacement amount of the platen roller 67 when it is driven at a low speed with 1-2 phase excitation in the case where the stepping motor having the feature shown in FIG. 2 is adopted to the driving mechanism for driving the platen roller 67 in the recording portion 63, and FIG. 5 shows an example of an image reproduced or recorded when the wholly black image signal is transmitted or received in the above-mentioned condition.
As apparent from FIG. 5, while a wholly black image signal is transmitted or received, the recorded image will nevertheless include white stripes due to the uneven feeding of the original and recording sheet for the above-mentioned reasons.
FIG. 6A shows current values of phase A and phase B in each quadrant when the stepping motor is driven with micro-steps, FIG. 6B shows a phase A current value, and FIG. 6C shows a phase B current value. FIGS. 7 and 8 show an example of a construction of PM-type stepping motor. The stepping motor includes a stator 80 and a rotor 81 which is spaced apart from the stator by a distance l. FIG. 7 shows a condition that the motor is stopped in a second phase (A, BX=N, AX, B=S) and FIG. 8 shows a condition that the rotor is stopped in a first phase (A=N, AX, B, BX=S). Here, S and N are polarities of a magnetic pole.
As mentioned regarding the above conventional facsimile system, a holding torque of the stepping motor when it is driven with first phase excitation differs from that when driven with second phase excitation. That is to say, when the stepping motor is driven with the second phase excitation, the rotor is shifted with strong force, whereas, when the stepping motor is driven with the first phase excitation, the rotor is shifted with weak force. Consequently, the vibration of the stepping motor is increased, generating the uneven rotation of the motor, thus forming the white stripes in the recorded image as shown in FIG. 5. This phenomenon will be emphasized when the stepping motor is rotated at a low speed i.e., in a high quality image mode (fine mode).
In order to solve such problem, a technique wherein a sheet feed for single scanning line is effected through two steps of the stepping motor has been proposed, for example, as disclosed in the Japanese Patent Publication No. 62-37866. According to this conventional technique, since sub-scanning drive is effected on the basis of a unit which corresponds to the sum of the moved distance of the rotor when the stepping motor is driven with the first phase excitation and the moved distance of the rotor when the stepping motor is driven with the second phase excitation, dispersion in distances between the scanning lines can be eliminated.
However, in the fine mode of a facsimile system, since delicate control is required, it is necessary to perform the sheet feed for a single scanning line through a single step of the stepping motor. In such a case, there will arise a problem wherein the holding torque of the stepping motor, when it is driven with first phase excitation, differs from that when driven with second phase excitation.
Further, in order to solve the above-mentioned problem, there has been proposed a micro-step drive technique that, when the sum of the current vectors exists on a circle as shown in FIG. 6A, necessary micro-steps are sought by decomposing the full-step (when the electric phase angle is 90.degree.), and a single step of the stepping motor is decomposed into a given number of micro-steps. According to this drive technique, the vibration of the stepping motor can be reduced, whereby the original and the recording sheet can be uniformly fed. However, since D/A converters, special control circuits and the like are required to realize this technique, the system was expensive and was large-sized.
Further, as shown in FIGS. 7 and 8, since the holding torque is proportional to the total value of the exciting currents flowing in the stator of the stepping motor, when it is assumed that the current flow in the first phase excitation is "1", the current flow in the second phase excitation will be .sqroot.2 (=1/.sqroot.2 +1/.sqroot.2). Consequently, the holding torque was increased accordingly, with the result that the vibration of and the uneven rotation of the stepping motor was not effectively eliminated even by the above-mentioned drive technique.
In order to solve this problem, a technique that the motor currents in the first and second phase excitations for a stepping motor are controlled to reduce or eliminate the difference between the torque generated in the first phase excitation and the torque generated in the second phase excitation has been proposed, for example, in the U.S. Pat. No. 4,642,544 or in the U.S. Ser. No. 148,690, now U.S. Pat. No. 4,857,817. In both cases, however, such proposal relates to the stepping motor alone and does not relate to the sheet feeding apparatus or the facsimile system at all. In addition, the latter proposes a technique that the current is changed by a resistor and the former proposes a technique that the current is controlled by controlling the duty of pulses; thus, these techniques made the construction complicated.
Furthermore, in the conventional facsimile systems, there was a further problem that, since the cutter for cutting the recorded sheet was normally driven by the stepping motor, noise was generated during the cutting operation of the recording sheet.